The present invention relates to a lithium ion secondary battery and particularly, to the improvement of a positive electrode.
Currently, the positive electrode active material of lithium ion secondary batteries used is typically a lithium-containing composite oxide such as LiCoO2. LiCoO2, however, has its limits in terms of capacity, safety, etc. As such, usually cobalt (Co) in LiCoO2 is partially substituted by other element(s) to improve the characteristics thereof.
Japanese Laid-Open Patent Publication No. Hei 6-168722, for example, proposes to partially substitute Co of LiCoO2 with magnesium (Mg) for cycle characteristic improvement. Japanese Laid-Open Patent Publication No. 2002-203553 proposes the partial substitution of LiCoO2 with at least one selected from the group consisting of aluminum (Al), chromium (Cr), vanadium (V), manganese (Mn) and iron (Fe) and with at least one selected from the group consisting of Mg and calcium (Ca) for battery capacity improvement as well as for preventing the battery temperature from rising during overcharge.
Moreover, Japanese Laid-Open Patent Publication No. 2000-123834 proposes to use raw materials with high purity to reduce the concentrations of impurities introduced during production, namely, iron (Fe), copper (Cu), sodium (Na), silicon (Si) and nickel (Ni) to not greater than 0.03%, 0.005%, 0.1%, 0.1% and 0.15%, respectively, for preventing the reduction of initial capacity.
In view of the above, the present inventors have studied the partial substitution of Co with a small amount of Mg and a trace amount of Al in order to improve the battery capacity and the battery safety against internal short-circuit and overcharge.
However, LiCoO2 in which Co is partially substituted by a small amount of Mg and a trace amount of Al has had defects such as the deterioration of storage characteristics and the generation of significant amount of gases such as CO and CO2, by dissolving of Mg during high temperature storage.
The present inventors have further conducted extensive studies thereon and found that, among the impurities contained in raw materials during production, sodium (Na) and potassium (K), even trace amounts thereof, have enormous influence on the above-mentioned problems. When Co sites are substituted by certain amounts of sodium and potassium in a crystal structure, the crystal structure is stabilized and the dissolving of Mg during high temperature storage is prevented. When the amount is too large, however, an oxide of sodium or potassium is produced to generate O2.
The amounts of impurities can be reduced by using raw materials with high purity. As is clear from the above, there are preferred amount ranges of sodium and potassium. However, the crystal structure is not stabilized by such amounts as are contained as impurities.
Accordingly, the present invention is intended to solve the above problem and provides a lithium ion secondary battery superior in high temperature storage characteristics and safety by using raw materials with high purity and separately adding certain amounts of sodium and potassium salts thereto to optimize the contents of sodium and potassium in a positive electrode active material (i.e. LiCoO2) in which Co is partially substituted by a small amount of Mg and a trace amount of Al.